In the field of nuclear magnet resonance imaging (MRI), it is known that certain atomic nuclei, e.g. .sup.1 H, .sup.19 F, .sup.31 P, have nuclear magnetic moments which, when placed in a static magnetic field, can only take up certain discrete orientations. Each of these orientations corresponds to a different energy state for the nucleus. Further, it is known that the application of radio frequency (RF) radiation to nuclei in a magnetic field can induce a transition in the energy state of nuclei from one level to another. Such a transition is known as nuclear magnetic resonance (NMR) and it occurs under certain identifiable conditions. Many of these conditions relate to the magnetic fields which are imposed on the nuclei.
A magnetic field is described in terms of flux and lines of flux. From the standpoint of an individual nucleus, there are three essential parameters which are characteristic of the magnetic field at the location of a nucleus. These are: 1) the magnitude of the field, 2) the direction of the field as indicated by the flux line through the nucleus, and 3) the gradient, or rate of change in magnitude, of the field at the location of the nucleus. Each of these parameters has its effect on NMR.
As stated above, nuclei assume certain discrete orientations in a magnetic field which may be subsequently changed by RF radiation. The RF radiation which is most effective for inducing such a change is a particular frequency which depends on the magnitude of the magnetic field at the location of the nucleus. This particular frequency, more familiarly known as the Larmor frequency, is equal to the angular frequency of precession of the nucleus spin vector about the direction of the magnetic field vector. When changes in the orientations of nuclei are induced by radiation applied at the Larmor frequency, the magnetic moments of the nuclei will generate signals which are characteristic of the nuclei. These signals are receivable by an RF antenna. Specifically, for medical applications, it is known that nuclei of various tissues will have different NMR responses. Further, it is known that particular parameters of the NMR response can be weighted using known mathematical techniques to more definitively identify the tissue.
As a practical matter, there are always many nuclei in the object to be imaged by NMR, and each nucleus will generate its own signal. However, as long as the nuclei are spinning in phase with each other, their individual magnetic moments are coherent and create a net moment. Insofar as NMR is concerned, it is known that nuclei, under the influence of a field gradient, will begin to defocus or depolarize after their energy states have been changed. Further, it is known that the rate at which the nuclei defocus is essentially proportional to the dominant field gradient. While it is important to recognize that selectable field gradients can be imposed to encode nuclei, these encoding gradients are controllable and are deliberately imposed for the specific purpose of generating a useable encoded NMR projection which can be received by an RF antenna. This condition, however, is to be distinguished from the condition caused by uncontrollable permanent field gradients which are caused by inhomogeneities in the field. It is these permanent, and basically inherent, field gradients which cause nuclei to spin out of phase with each other, i.e. become defocused. When the nuclei become defocused, a condition is created wherein the NMR signal is not detectable. The object then, is to maintain a focused, in-phase, alignment of the nuclei which will give a net moment signal that can be detected.
A solution to this problem is to create a magnetic field wherein there are no permanent field gradients, or wherein the gradients are so small they will cause negligible defocusing effects on the detectable signal. To this end, numerous inventions have created homogeneous magnetic fields having regions where there are effectively no field gradients. The only field gradients in the homogeneous region will then be those which are intentionally imposed to impart an encoding pattern. This, however, is an idealized homogeneous field system which, in practice, is very difficult to achieve. In some systems a high degree of homogeneity is achievable. The expense, however, is great.
Typically, NMR systems which use homogeneous magnetic fields require exceedingly large high-powered magnets. The systems are extremely bulky and cumbersome and are, therefore, effectively immobile. Further, the region of homogeneity is substantially surrounded by the magnet. Stated differently, the homogeneous field is an internal field. Nevertheless, due to the fact that homogeneous fields are capable of generating high quality images, NMR systems which generate homogeneous fields are widely used.
Because NMR is capable of yielding extremely valuable information on the structure of molecules, one very important application for NMR is in procedures for the noninvasive clinical imaging of the human body. As an example, U.S. Pat. No. 4,498,048 which issued to Lee et al. for an invention entitled "NMR Imaging Apparatus" is typical of a commonly used medical MRI system. In addition to its bulk and immobility, a significant aspect of systems such as the Lee et al. system is that it must substantially surround the object to be imaged. A notable exception in this field, however, is U.S. Pat. No. 5,117,188 which issued to Ohkawa for an invention entitled "Quasi-Open Magnet Configuration for use in Magnetic Resonance Imaging". While systems such as Ohkawa avoid surrounding the imaging region, the generation of an exterior homogeneous field requires the juxtaposition of large and strong opposing sources. Again, as with the other systems which generate homogeneous fields, Ohkawa's system is bulky and effectively immobile.
In light of the above, the present invention has recognized that external nonhomogeneous fields can be generated by relatively small magnet configurations and used to advantage for MRI systems. Such MRI systems must, however, accommodate the characteristics of the external nonhomogeneous field. To do this, there are three particularly important considerations. As more fully discussed below, these are: 1) the rate at which nuclei defocus and can be refocussed, 2) the mechanism of signal to noise ratio (SNR) compensation, and 3) the diffusion of nuclei as they defocus. As further recognized by the present invention, the adverse effects of both the second and third considerations can be overcome by proper use of the first consideration.
An exterior nonhomogeneous flux field has an inherent dominant field gradient, denoted (G.sub.z), which acts in a direction, denoted (z), which is substantially perpendicular to surfaces of constant field magnitude, denoted (B.sub.0). Such surfaces, in conjunction with selected Larmor frequencies will establish an image plane which is exterior to the static magnet being used to generate the nonhomogeneous field. As alluded to above, it is the dominant and permanent field gradient G.sub.z in a nonhomogeneous field which defocuses the nuclei and causes the NMR signals from the image plane to deteriorate. The dominant gradient may also require wider system bandwidths, thus allowing more noise power to corrupt the NMR signal. Finally the presence of a large G.sub.z cause incoherent dephasing that is irretrievable.
It is known, however, that controllable and selectable flux gradients G.sub.x and G.sub.y (where x and y are perpendicular to z and transverse to the image plane) can be imposed to encode nuclei. In an exterior nonhomogeneous field, however, the gradient G.sub.z dominates the other gradients G.sub.x and G.sub.y. This dominance by G.sub.z has its implications. For instance, it is known that once nuclei in the field have been aligned by a pulse of RF radiation into an orientation wherein their net magnetic moment signals are in phase, the nuclei will defocus more quickly from the aligned orientation as G.sub.z is increased. However, as G.sub.z is increased, the nuclei will also refocus at the same faster rate back into the aligned orientation if refocussed by RF radiation. The implication of this, which is recognized by the present invention, is that the increased rate of defocus and refocus can be effectively used to shorten the time interval between nuclei refocussing pulses. From the above, it is to be appreciated that with a very large dominant G.sub.z the time for refocus and defocus is very short. For example, for a G.sub.z equal to 400 Gauss/cm the time for nuclei to refocus and defocus in a one millimeter thick slice is less than 10 microseconds. Realistically, the time interval is so short that it is practical to obtain only one independent measurement of the net magnetic moment in the interval between refocussing pulses.
Restated from above, the fact that nuclei defocus very quickly in an external nonhomogeneous field is offset by the fact that defocused nuclei can be refocussed just as quickly. Therefore, although a useable net magnetic moment is short lived, it can be regained quickly. With this capability, it has been recognized that the effective sampling rate, i.e. the rate at which independent measurements of a net magnetic moment can be made, is on the order of the periodic rate at which nuclei can be refocussed.
The second problem with the dominant gradient G.sub.z in the external nonhomogeneous field is related to the fact that a wider system bandwidth is required to accommodate the wider range of Larmor frequencies. This results in a decreased signal to noise ratio (SNR) due to the extra noise power associated with the wider bandwidth. This decrease in SNR may be offset, however, by the fact that an independent measurement of the net magnetic moment NMR response can be accomplished more quickly in the higher bandwidth cases. Accordingly, with standard signal averaging techniques, an accelerated averaging rate afforded by the high gradient G.sub.z, can be used to efficiently compensate for the wider noise bandwidth.
As also indicated above, the third issue of molecular diffusion inherent with nonhomogeneous magnetic fields can be overcome by the rapid refocussing rate which is made possible by a high gradient G.sub.z. Simply stated, molecular diffusion is a phenomenon associated With the random motion of defocusing nuclei in a large field gradient G.sub.z. Although the random displacement of the diffusing nuclei may be small, the different Larmor frequencies experienced in a large field gradient causes the nuclei to defocus randomly. The real difficulty of molecular diffusion is due to the fact that the nuclei will also refocus randomly. Diffusion related dephasing is, therefore, effectively an irretrievable process. That is, of course, unless the process can somehow be controlled. To this end, the present invention has recognized that the high refocussing repetition rate, afforded by a high field gradient G.sub.z, allows nuclei to be refocussed before diffusion proceeds to the point where the previous orientation of nuclei is irretrievable.
Non homogeneous field MRI systems which have heretofore been proposed have neither taught nor suggested ways in which to exploit the unique characteristics of an external nonhomogeneous flux field for the purposes of MRI. For example, both U.S. Pat. No. 5,023,554 which issued to Cho et al. for an invention entitled "Fringe Field MRI", and U.S. Pat. No. 4,379,262 which issued to Young for an invention entitled "Nuclear Magnetic Resonance Systems", disclose the use of a nonhomogeneous magnetic field for MRI and suggest the field may be external to the magnet. In particular, Cho et al. disclose a fringe field having strengths substantially greater than two Tesla. Importantly, neither Cho et al. nor Young, disclose a system or method which uses accelerated refocussing to overcome the above described mechanisms concerning defocusing, the rapid deterioration of SNR or diffusion under the influence of a large dominant gradient G.sub.z. Such a system or method is particularly required to compensate for the weaker signals and more dominant gradients present in lower field strength devices (e.g. &lt;2 Tesla).
The refocussing of nuclei necessitated by field inhomogeneities has been considered before. For example, U.S. Pat. No. 4,656,452 which issued to Bendel for an invention entitled "Method to eliminate the Effects of Magnetic Field Inhomogeneities in NMR Imaging and Apparatus therefor" discloses a method whereby nuclei in a magnetic field, which have been pulsed with RF radiation, are repetitively refocussed after they begin to defocus. Bendel, however, does not teach or suggest using such a method with an external magnetic field where G.sub.z dominates the controllable gradients G.sub.x and G.sub.y. Therefore, and more specifically, Bendel does not address the problems inherent with establishing and identifying a portion of an external magnetic field where effective MRI can be accomplished using an external nonhomogeneous magnetic field. As another example, U.S. Pat. No. 4,542,343 which issued to Brown for an invention entitled "Computer-Controlled, Portable Pulsed NMR Instrument and Method of Use wherein Depolarization is Ultrashortened using Pseudo Randomly Spaced and Oriented RF Depolarization Pulses" discloses a device and method for using NMR responses to determine the fluid-flow properties of a rock sample. Although Brown discloses use of an external magnetic field, and the use of rapid refocussing techniques to overcome the inadequacies of magnetic field inhomogeneities, Brown is not concerned with imaging. Consequently, Brown neither teaches nor discloses how the components and capabilities of such a system can be used for MRI.
In light of the above it is an object of the present invention to provide a diagnostic MRI device which is compact and remotely positionable relative to the object being imaged. It is another object of the present invention to provide an MRI device which does not confine the measurement surface within the structure of the magnet during an MRI procedure. Another object of the present invention is to provide an MRI device which is configured to establish a substantially flat measurement surface in an external nonhomogeneous magnetic field. Still another object of the present invention is to provide an MRI device which improves the effective SNR in an NMR response by accelerated signal averaging using a sampling rate that is on the order of the periodic rate of refocussing. Yet another object of the present invention is to provide a device which effectively uses an external nonhomogeneous static magnetic flux field for MRI. Another object of the present invention is to provide an MRI device which allows for a trade off between a higher SNR and increased speed in image acquisition. Still another object of the present invention is to provide a method for using an MRI device which accomplishes the objects set forth above. Another object of the present invention is to provide a method for generating images using a plurality of NMR responses which are acquired at a rate that is proportional to the field gradient. An object of the present invention is to provide an MRI device and a method for NMR imaging that is easy to use and comparatively cost effective.